1. Field of the Invention
The present invention relates to a transmission power control apparatus, and more particularly, to an apparatus for controlling a power of a transmission signal, which is incorporated in a transmission section of a Time-Division Multiple Access (TDMA) radio transmission/reception apparatus of a base station or mobile station in a digital mobile communication system employing an amplitude modulation method.
2. Prior Art
FIG. 1 shows a conventional transmission power control apparatus. In FIG. 1, the reference numeral 1 denotes an input terminal to which a high-frequency input signal is provided, 2 a variable attenuator for attenuating the input signal in accordance with an attenuation control signal V.sub.APC, 3 a power amplifier for amplifying a power or amplitude of a signal from in the attenuator 2, 5 an output terminal for outputting the attenuated and amplified signal as a power controlled transmission signal, and 5 a branch circuit for branching a part of the attenuated and amplified signal.
The reference numeral 6 denotes a detection circuit for detecting a direct current (DC) level of a signal branched by the branch circuit 5 to produce a detection voltage V.sub.DET which is proportional to the level of the transmission signal. The numeral 7 denotes a reference voltage source for providing a reference voltage V.sub.REF, 8 an error amplifier for comparing the detection voltage V.sub.DET with the reference voltage V.sub.REF to generate an error voltage V.sub.ERR corresponding to the difference between the detection and reference voltages V.sub.DET and V.sub.REF, and 9 a time constant circuit for controlling a time constant for the error voltage V.sub.ERR to provide a voltage as the attenuation control signal V.sub.APC to be supplied to the attenuator 2. The reference numeral 10 denotes a control circuit for controlling the reference voltage source 7 and time constant circuit 9 to vary the values of the reference voltage V.sub.REF and time constant by providing reference voltage and time constant control signals S.sub.10--7 and S.sub.10--9 thereto, respectively.
in the prior transmission power control circuit shown in FIG. 1, the reference voltage V.sub.REF output from the reference voltage source 7 is used to set the power level of the transmission signal. That is, the high-frequency input signal is attenuated and amplified through the attenuator 2 and the amplifier 3 and then is output from the output terminal 4, while it is branched by the branch circuit 4 and then is converted into the DC detection voltage V.sub.DET at the detection circuit 6. The detection voltage V.sub.DET is compared with the reference voltage V.sub.REF by the error amplifier 8 to create the error voltage V.sub.ERR. The error voltage V.sub.ERR is supplied as the attenuation control signal V.sub.APC via the time constant circuit 9 to the variable attenuator 2 to adjust the attenuation amount of the signal such that the detection voltage V.sub.DET becomes the same as the reference voltage V.sub.REF. Accordingly, the transmission signal from the output terminal 4 will be set to have a predetermined power level determined by the reference voltage V.sub.REF.
In the prior art, in order to perform transmission power control with high accuracy, such a feedback control as described above is needed. However, in handling a signal with an amplitude variation, or an amplitude modulated signal, a time constant of the feedback control has to be increased to such an extent that it does not respond to the amplitude information of the modulated signal, thereby the amplitude factor of the modulated signal is not suppressed by the feedback control. A transmission signal subjected by a .pi./4DQPSK modulation can be taken as an example signal with the amplitude information.
On the other hand, when the reference voltage V.sub.REF should be changed by the control circuit 10, the feedback loop have to follow the change of the reference voltage V.sub.REF without a substantial delay. To cope with this, the time constant is required to be made smaller. This necessity exists irrespective of types of modulation and/or transmission, and/or whether the transmission power control circuit is incorporated in a base station or in a mobile station of a mobile communication system.
Therefore, opposite requirements exist in the feedback control of the transmission power control circuit.
Taking as an example, a transmission power control circuit in a transmission section of a TDMA radio transmission/reception in a base station, the above-mentioned problems will be described in detail by reference to FIGS. 2(A)-2(C), 3, and 4(A), 4(B).
FIG. 2(A) shows a waveform of the reference voltage V.sub.REF applied from the reference voltage source 7 to the error amplifier 8. The voltage V.sub.REF is varied in accordance with the control signal S.sub.10--7 provided from the control circuit 10 as shown in FIG. 2(B). FIG. 2(C) illustrates a waveform of the attenuation control voltage V.sub.APC provided from the time constant circuit 9 to the variable attenuator 2. In FIG. 2(B), the logical high and low levels of the control signal S.sub.10--7 correspond to the time constant to be lowered and heightened, respectively.
FIG. 3 is a diagram showing a first spectrum SP.sub.1 in a case where transmission power levels are different for respective slots and a second spectrum SP.sub.2 in a case where they are the same for all the slots. FIGS. 4(A) and 4(B) show conventional detailed circuit diagrams of the time constant circuits 9, where the circuits utilize an analog switch SW and diodes D.sub.1 and D.sub.2, respectively to change the time constant.
In particular, the transmission section of the TDMA radio transmission/reception apparatus installed in the base station is generally constituted such that different output powers can be set for individual transmission slots because distances therefrom to respective mobile stations within a radio zone are not substantially constant. Due to this condition, it is needed to set the reference voltage V.sub.REF for the individual transmission slots. Further in the base station, since errors in the transmission power levels lead to errors in a service area, high accuracy of the power levels is required. Moreover, by such a power level control as described above, neither an amplitude modulated signal in a current channel or slot nor channels adjacent to the current slots must be affected.
To this end, when the slots are changed, it is required that a transmission power level of the next slot is quickly and accurately attained and that the adjacent channels are prevented from being interfered with expansion of the transmission wave spectrum of the transmitter. Accordingly, the time constant of the feedback control needs to be made so small that it is followed at a high speed to the reference voltage V.sub.REF which smoothly varies as shown in FIG. 2(A). With a view to satisfying these two contradictory requirements, there is provided a method in which, as shown in FIG. 2(B), the time constant is switched to small only during a power control time period, or during the time constant control signal S.sub.10--9 is high, when the slots are to be changed.
As one means for realizing the above method, a serial connection of the analog switch SW and a resistor R1 is connected in parallel with a resistor R2 as shown in FIG. 4(A). The switch SW is turned on only when the control signal S.sub.10--9 is high as shown in FIG. 3(B) in order to reduce the time constant at the time constant circuit 9. In this case, however, there are drawbacks that high-frequency factors at the front and back edges of pulses included in the control signal S.sub.10'7 leak through an internal capacity of the analog switch SW to the attenuation control voltage V.sub.APC, and this ends up generating noise as shown in FIG. 2(C), whereby adjacent channels are affected as shown in FIG. 3.
Another means for realizing the above method of changing the time constant is shown in FIG. 3(B), in which diodes D.sub.1 and D.sub.2 are used instead of the analog switch SW to switch the time constant. In this case, however, unless there is generated a voltage greater than a forward drop voltage (in the case of a Shottky barrier diode, about 0.1 V), the diodes cannot be turned on, and therefore this requires a certain device for turning on/off the diodes. Thus, there is a difficulty in controlling transmission power levels for individual slots.